The present invention relates to chromophores with very large two-photon absorption cross-sections.
Two-photon or multiphoton absorption occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium, with the former being more common. For a given chromophore, these absorption processes take place at wavelengths much longer than the cut-off wavelength of its linear (single-photon) absorption. In the case of two-photon absorption (TPA), two quanta of photons may be absorbed from a single light source (degenerate TPA) or two sources of different wavelengths (non-degenerate TPA). Although multiphoton absorption processes have been known since 1931, this field remained dormant largely due to the lack of TPA-active materials with sufficiently large cross-sections. In the mid-1990s, several new classes of chromophores exhibiting very large effective TPA cross-section ("sgr"2xe2x80x2) values were reported. In conjunction with the increased availability of ultrafast high-intensity lasers, the renewed interest has not only sparked a flurry of activities in the preparation of novel dye molecules with enhanced "sgr"2xe2x80x2 values, but also many previously conceived applications based on TPA process in photonics and biophotonics are now enabled by these new chromophores. It is important to recognize the following features of two-photon materials technology: (a) upconverted emission, whereby an incident light at lower frequency (energy) can be converted to an output light at higher frequency, for instance, IR to UV-Vis upconversion; (b) deeper penetration of incident light; (c) highly localized excitation allowing precision control of in-situ photochemical events in the absorbing medium, thereby minimizing undesirable activities such as photodegradation or photobleaching; (d) fluorescence when properly manipulated allows information feedback. It is anticipated that further ingenious utilization of these basic characteristics will lead to practical applications other than those already emerged in such diverse areas as fluorescence imaging, data storage, eye and sensor protection, microfabrication of microelectromechanical systems (MEMS), photodynamic therapy, etc.
In U.S. Pat. No. 5,770,737, Reinhardt et al disclose asymmetrical dyes with large two-photon absorption cross-sections and in U.S. Pat. No. 5,859,251, Reinhardt et al disclose symmetrical dyes with large two-photon absorption cross-sections. The asymmetrical dyes have the structure Acceptor-Core-Donor, and the symmetrical dyes have the structures Acceptor-Core-Acceptor and Donor-Core-Donor. In U.S. Pat. No. 6,300,502, Kannan et al disclose multi-branched TPA chromophores with 4, 5 or 6 branches from the core, thereby increasing the number density, i.e., the number of TPA-active subunits within the individual chromophore molecules.
Accordingly, it is an object of the present invention to provide new multi-branched TPA chromophores.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the present invention there are provided novel TPA chromophores having the structure
Qxe2x80x94(Lxe2x80x94Z)x,
wherein x is 2 or 3, wherein Q is selected from the group consisting of 
wherein L is 
wherein R is an alkyl group having 1 to 20 carbon atoms, and wherein Z is selected from the group consisting of 